function my_test_function() {

    $started = get_transient( 'wpf_woo_started_16133' );

    if ( ! empty( $started ) ) {
        return;
    } else {
        set_transient( 'wpf_woo_started_16133', true, HOUR_IN_SECONDS );
    }

    // Any code here should only happen once, but it happened twice

}

my_test_function();

my_test_function();